Network densification—the practice of deploying more radio access nodes into a geographical area—is being considered as a cost and bandwidth-effective method to increase wireless network capacity. In a dense heterogeneous network, small-cell systems, (also known as femtocells) co-exist and cooperate with high-power macrocells to serve users' traffic demands. To mitigate the interference problem in such a dense deployment, many advanced management techniques have been proposed, including those that use antenna directionality. By using directional antennas, small-cell network nodes can focus energy in only the intended direction, thereby creating less interference between links and more potential for spatial reuse. Nevertheless, bringing these techniques to practice has been challenging